Many liquid crystal display devices which are currently in practical use utilize polarized light. Such display devices have a problem in that polarizing plates which are used for generating polarized light lower the efficiency of light utilization.
In order to obtain an enhanced displaying performance by improving the efficiency of light utilization of a liquid crystal display device, it has been proposed to utilize a scattering display mode by using scattering-type liquid crystal. In the scattering display mode, a voltage which is applied across a liquid crystal layer causes the liquid crystal layer to switch between a transmitting state in which light is transmitted and a scattering state in which light is scattered, and displaying is performed by utilizing this. A display device which utilizes such a display mode does not need polarizing plates, so that its efficiency of light utilization can be enhanced. There is also an advantage in that there is little viewing angle dependence, and so on.
As an example of a display device utilizing the scattering display mode, Patent Document 1 discloses a retroreflection display device in which a retroreflection plate is disposed at the rear face side of a liquid crystal layer (i.e., the opposite side of the liquid crystal layer from the viewer), for example.
Hereinafter, with reference to the drawings, the operation principles of a retroreflection display device will be described. FIGS. 1(a) and (b) are diagrams for explaining a black displaying state and a white displaying state of a display device.
As shown in FIG. 1(a), while the liquid crystal layer 1 is controlled to be in a transmitting state, incident light 3 from a light source 5 which is external to the display device travels through the liquid crystal layer 1, and thereafter is reflected by a retroreflection plate 2 in the direction in which the light has entered (reflected light 4b). Therefore, the light from the light source 5 does not reach the viewer 6. At this time, it is an image of the eye(s) of the viewer himself or herself that reaches the viewer 6 from the display device, whereby a “black” displaying state is obtained.
On the other hand, as shown in FIG. 1(b), while the liquid crystal layer 1 is controlled to be in a scattering state, the incident light 3 from the light source 5 is scattered in the liquid crystal layer 1. In the case where the liquid crystal layer 1 is a forward scattering-type liquid crystal layer, the scattered light is reflected by the retroreflection plate 2, furthermore travels through the liquid crystal layer 1 in a scattering state, and goes out in the viewing direction (reflected light 4w). Since the retroreflectivity of the retroreflection plate 2 is counteracted by the scattering in the liquid crystal layer 1, the incident light 3 does not go back in its incident direction. As a result, a “white” displaying state is obtained.
By performing displaying based on such operation principles, it is possible to realize white-black displaying without using polarizers. Therefore, a highly bright reflection liquid crystal display device can be realized, free from the influence of lowering of the efficiency of light utilization due to use of polarizers.
Patent Document 2 filed by the Applicant proposes, as the retroreflection plate 2, using a corner cube array in which corner cubes are placed in a two-dimensional array, each corner cube being composed of three faces which are orthogonal to one another. Since the corner cube array has a high retroreflectance, the amount of unnecessary light that reaches the viewer 6 in a black displaying state is reduced, whereby the displaying contrast ratio can be further improved.
Next, with reference to the drawings, the specific construction of a retroreflection liquid crystal display device will be described. As an example, a display device will be described which is constructed so that a retroreflection plate is disposed on the outer side of a rear substrate (i.e., the opposite side of the rear substrate from the liquid crystal layer). The construction of such a display device is disclosed in Patent Document 3, for example. Note that, in the present specification, between two opposing substrates, the substrate which is located on the viewer's side will be referred to as the “front substrate”, whereas the substrate located on the opposite side from the viewer will be referred to as the “rear substrate”.
FIG. 2(a) is a plan view showing wiring lines and electrodes being present on the rear substrate of a retroreflection liquid crystal display device. FIG. 2(b) is a diagram for describing the construction of a retroreflection liquid crystal display device, which is a schematic cross-sectional view of the retroreflection liquid crystal display device along line II-II′ in the plan view of FIG. 2(a).
The display device 100 includes a front substrate 10 and a rear substrate 12 disposed so as to oppose the front substrate 10. Between the substrates 10 and 12, a scattering-type liquid crystal layer 1 which is capable of taking a scattering state or a transmitting state is provided.
On a face of the rear substrate 12 closer to the liquid crystal layer 1, a plurality of thin film transistors (TFTs: Thin Film Transistors) 13 functioning as switching elements, a plurality of pixel electrodes 16, source lines 14 which are connected to the pixel electrodes 16 via the TFTs, gate lines 15 for selectively driving the thin film transistors 13, and the like are formed. The pixel electrodes 16 are formed by using an electrically conductive material which transmits light, e.g., ITO (indium tin oxide). As shown in FIG. 2(b), the pixel electrodes 16 are disposed so as to be spaced apart, thus defining pixels, each of which is one unit of image displaying. On the other hand, generally speaking, wiring lines such as the source lines 14 and the gate lines 15 are formed by using a metal material which shields light, e.g., tantalum. Although not shown, the wiring lines 14 and 15 are respectively connected to a source driver and a gate driver in a driving circuit which is provided on the rear substrate 12. The wiring lines 14 and 15 are disposed at predetermined intervals 22 and 23 from each pixel electrode 16 in order to ensure that the parasitic capacitances between themselves and the pixel electrode 16 will not be too large. The intervals 22 and 23 between each pixel electrode 16 and the wiring lines 14 and 15 are e.g. about 3 μm. On the opposite face of the rear substrate 12 from the liquid crystal layer 1, the retroreflection plate 2 is provided.
On the front substrate 10 a counter electrode 18 composed of color filters 19, a black matrix 20, and a transparent conductive film is provided. The color filters are provided for the respective pixels. The black matrix is disposed between adjoining pixels and in the neighborhood of the display region, as necessary, so as to shade the wiring lines 14 and 15 and the thin film transistor 13.
In the display device 100, by controlling the voltage which is applied between the counter electrode 18 and the pixel electrode 16, it becomes possible to switch the liquid crystal layer 1 between a scattering state and a transmitting state.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-107519    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2003-066211    [Patent Document 3] Japanese Laid-Open Patent Publication No. 11-15415